It is known that 4,4' bisphenol sulfone (4,4' dihydroxydiphenyl sulfone) can be synthesized from phenol and sulfuric acid by reactions that may be depicted by the following equations: ##STR1## The end product of the reactions is a mixture of the 4,4' and 2,4' isomers of bisphenol sulfone. As these favored reactions proceed, there occur also side reactions which produce small amounts of quinone-type coloring agents and other impurities which have not been fully identified.
The formation of both 2,4' bisphenol sulfone by-product and side-product impurities is apparently inevitable and is certainly undesirable. Not only does their formation reduce the yield of the 4,4' bisphenol sulfone, but also the presence of these products greatly reduces the utility of the recovered bisphenol sulfone product. A particularly useful application of 4,4' bisphenol sulfone is as a monomer in the production of high molecular weight polymers such as polyphenylsulfone resins. However, to be maximally useful in such polymerizations, 4,4' bisphenol sulfone must be substantially free of its 2,4' isomer and otherwise be of high purity. Polymers made from 4,4' bisphenol sulfone having a significant amount of 2,4' isomer generally neither reach the high molecular weights nor exhibit the good temperature resistance and mechanical properties that are common to polymers made from purer 4,4' bisphenol sulfone. Accordingly, it is most desirable for a commercial process that formation of 2,4' bisphenol sulfone and other impurities be minimized in the synthesis of 4,4' bisphenol sulfone.
One technique that has been proposed for increasing the yield of 4,4' bisphenol sulfone involves distilling from the reaction medium the water formed in the reactions of phenol with sulfuric acid (Australian Patent No. 165,526). Entraining the evolved water in a solvent has also been proposed (e.g. U.S. Pat. No. 3,065,275). The removal of water by either of these methods results in a forward equilibrium shift in the reactions, depicted by equations I and II above, thereby increasing the production of 4,4' bisphenol sulfone. However, simply altering the equilibrium of the reactions between phenol and sulfuric acid also proportionately increases the production of undesirable 2,4' bisphenol sulfone by-product.
It is known that, in the liquid phase, the 4,4' and 2,4' isomers of bisphenol sulfone exist in a dynamic equilibrium which may be depicted by the following equation: ##STR2##
Removal of 4,4' bisphenol sulfone from the liquid phase results in a forward shift in the dynamic equilibrium, with the 2,4' isomer rearranging to the 4,4' isomer to reestablish the equilibrium concentrations. In other words, by removing 4,4' bisphenol sulfone from the liquid phase, the amount of 2,4' isomer by-product can be decreased while the production of the desired 4,4' bisphenol sulfone is increased.
One method that has been proposed to increase the yield of 4,4' bisphenol sulfone, by taking advantage of the liquid phase equilibrium, is described in U.S. Pat. No. 3,297,766. The described method involves precipitating 4,4' bisphenol sulfone from a conventionally formed isomer mixture, then recycling the remainder of the mixture to the reaction medium, with additional sulfuric acid and phenol reactants, whereupon some of the returned 2,4' isomer is said to rearrange to 4,4' bisphenol sulfone. This method, however, does not achieve satisfactorily high yields of 4,4' bisphenol sulfone until after the recycle is repeated several times.
Another method that is said to produce 4,4' bisphenol sulfone in high yield with minimal amounts of 2,4' isomer is described in U.S. Pat. No. 4,162,270. The described process involves reacting phenol with sulfuric acid in the presence of a solvent which is progressively removed from the reaction medium as the reaction proceeds. Removal of the solvent, it is said, causes the produced 4,4' bisphenol sulfone to precipitate and 2,4' isomer to rearrange to 4,4' bisphenol sulfone to reestablish the liquid phase equilibrium. The proposed method requires the complete removal of the solvent by vacuum distillation. Accordingly, this method is unattractive due to the expense and inconvenience of using vacuum and additional distillation equipment. The method is also cumbersome in requiring the use of a second solvent feed to recover from the reactor the precipitated product which may be an almost intractable solid mass.
Yet another proposed method for effecting rearrangement of 2,4' bisphenol sulfone by-product to the desired 4,4' bisphenol sulfone is described in United Kingdom Patent Application No. 2,030,566. In the described method, after a crystallized isomer mixture of bisphenol sulfone is formed, it is dissolved in an organic solvent and heated with an acidic catalyst to cause isomer rearrangement. This two step method of synthesizing higher proportions of 4,4' bisphenol sulfone, however, is inefficient due to the prolonged periods of heating and the possible production of additional side reaction impurities during the catalyzed isomer rearrangement step.